We investigate the growth of B-doped carbon nanotubes combining experimental and theoretical techniques. Electron microscopy observations and electron diffraction patterns reveal that B doping considerably increases the length of carbon tubes and leads to a remarkable preferred zigzag chirality. These findings are corroborated by first-principles static add dynamical simulations which indicate that, in the zigzag geometry, B atoms act as a surfactant during growth, preventing tube closure. This mechanism does not extend to armchair tubes, suggesting a helicity selection during growth.